In general, an RFID system consists of one or more tags, a tag reader, and a host computer system. Tags are devices that can come in many sizes and form factors, but are usually small and lightweight. Tags are commonly used as portable data devices that wirelessly communicate with RFID readers at distances ranging from a few millimeters to several meters. The information stored in a tag can be used, for example, to identify an individual or object carrying the tag.
RFID technology is used in a variety of applications because of its convenience and flexibility. An example application for RFID technology is a building security system. As part of a building security system, RFID systems are used to grant access only to individuals carrying authorized tags (or cards). When an individual places their card in the vicinity of the reader, the reader interrogates the card and obtains identification information stored in the card. After further processing, the reader communicates the individual's identification ("ID") code to a host computer in the security system. If the ID code received by the host computer system is authorized, the door is unlocked to permit access to the building.
RFID systems are also used to detect specific items and link those items with other information and events. RFID systems can be used, for example, to track products being built in a factory, to trigger manufacturing steps to occur, to assist in inventory control, etc. Read-only tags are ones in which the data is programmed once, and the tag only sends the stored information to the reader. Read-write tags have the ability to be reprogrammed to suit the needs of the application. Therefore, read-write tags can be used as portable databases, eliminating the need for central databases.
Most RFID tags contain an integrated circuit ("IC") to store and process data, and to perform communication functions. RFID tags also contain an electrode, which is used as the radio frequency interface with the reader. The IC requires power to operate, which can be supplied by a battery. Most applications, however, require tags to be small and inexpensive, so batteryless, or "passive", tags are in very wide use. Passive tags receive energy from the radio frequency ("RF") field generated by a reader, and the IC converts the RF to direct current ("DC") operating power for itself. Once operating, the IC communicates with the reader, which has an electrode system for transmission and reception of signals. Power and data are transferred between tag and reader through one or more electrodes in each device.
Some tag-reader systems communicate via magnetic fields, while other types of systems communicate via electric fields. Electric field tags offer advantages in cost, size, weight and flexibility compared with magnetic field tags. Many applications demand small, compact and inexpensive readers, as well. Shrinking the size of electric field RFID readers, however, presents unique design challenges. Without addressing these challenges, reader performance is significantly impaired.
FIG. 1A is an example of an electric field RFID system 10 having a compact RFID device 12. The RFID device 12 is composed of two basic elements, an exciter electrode (e.g., antenna, plate, etc.) 14 and electronic circuitry 16. The RFID device 12 may be any part of an RFID reader system containing tag excitation circuitry, such as, a tag reader, a tag writer, a tag reader/writer, a tag excitation device (in which the circuitry that performs the tag reading function is located in a separate unit), or any combination thereof. The exciter electrode 14 is a sheet of electrically conductive material. The electronic circuitry 16 contains all of the functional circuitry required to drive the exciter electrode 14, communicate information between a tag 20 and the RFID device 12, and exchange information with a host computer system 22 via an input/output ("I/O") cable 18. Power is provided to the RFID device 12 by the host computer system 22 via the I/O cable 18. The electronic circuitry 16 is commonly assembled on a substrate 17 comprised of a dielectric material, such as, epoxy glass printed circuit board (PCB). Alternatively, the substrate 17 may be made of a wide variety of materials, such as, polymer sheets or films, paper or cardboard, ceramic, etc. Components used in electronic circuitry 16 are interconnected by conductors on substrate 17. The conductors are formed of metals, metal foil, metal film, electrically conductive inks or paints, etc., and may be constructed using any suitable means, such as deposition and etching.
FIG. 1B is a side pictorial view/schematic diagram of the RFID system of FIG. 1A, which represents a monopole electric field RFID system. An exciter voltage source 30 generates a high alternating current ("AC") voltage that is connected to the exciter electrode 14. The exciter electrode 14, driven by the exciter voltage source 30, causes an AC electric field to be radiated outward toward the tag 20. When the tag 20 is close enough to the exciter electrode 14, and sufficient energy is coupled, the tag 20 then begins to function. This causes a small displacement current 32 to flow into the tag 20. Displacement current is that which flows through a dielectric when a time-varying potential exists across the dielectric. Current 32 flows through the tag 20, a common impedance path 34 (e.g., earth ground), and an RFID device reference connection 41, ultimately returning to the exciter voltage source 30 at the exciter voltage source return node 42. Therefore, current 32 provides operating energy for the tag 20. Relatively high voltage levels are required on the exciter electrode 14 in order to produce an adequate magnitude of current 32 when the tag 20 is at long distances from the exciter electrode 14. A receive electrode (not shown) is often located on or near the exciter electrode 14 for the purpose of receiving signals from tags.
It should be noted that FIG. 1B is not drawn to scale, that is, the tag 20 is typically positioned at a much greater distance from the exciter electrode 14 than is the electronic circuitry 16. Parasitic displacement current 76 flows from the exciter electrode 14 to other impedances that are broadly distributed in the environment surrounding the RFID device 12 and common to the RFID device reference connection 41. Parasitic displacement current 76 is due to stray capacitance, and is generally very small in magnitude. In FIG. 1B, the sum of current 32 and current 76 is shown as current 92 as it flows back to RFID device 12 through common impedance path 34 and RFID device reference connection 41.
In FIG. 1B, trace 36, trace 40 and sensitive component 38 (collectively referred to as "circuit elements") provide a simple representation of circuitry on the electronic circuitry 16. Because of their close proximity, significant capacitance exists between the exciter electrode 14 and the circuitry disposed on the electronic circuitry 16. Smaller separations between the exciter electrode 14 and the electronic circuitry 16 increase this capacitance. The exciter voltage source 14 is also located on the electronic circuitry 16, and the exciter voltage source return node 42 is common with many elements on the electronic circuitry 16 (e.g., circuit ground). Because of the AC potential difference between the exciter electrode 14 and circuit elements 36, 38 and 40, displacement current flows through the dielectric space between the exciter electrode 14 and the electronic circuitry 16. For illustration purposes, this displacement current is represented by lumped currents 44, 46, 48, 50, and 52, which flow respectively through lumped capacitances 54, 56, 58, 60 and 62. It will be appreciated by those skilled in the art that these capacitances and displacement currents are actually distributed over the entire area that is common to the exciter electrode 14 and the electronic circuitry 16, and are not necessarily discrete elements as illustrated in FIG. 1B.
Displacement currents 44, 46, 48, 50 and 52 respectively become conduction currents 64, 66, 68, 70 and 72 in circuit elements 36, 38 and 40 as the currents return to exciter voltage source 30 at the exciter voltage source return node 42. Because of the high exciter voltage and close spacing, the displacement currents (and therefore the resulting conduction currents) may become relatively large. As shown in FIG. 1B, displacement current 44 is injected into trace 36, becomes the conduction current 64, which in turn flows through sensitive component 38, and then into trace 40. Therefore, displacement current 44 causes noise voltage 69 to be developed across sensitive component 38; introducing signals that significantly reduce the performance of RFID device 12.
FIG. 1B further illustrates that displacement current 66 may be injected directly into sensitive component 38. As the distance from the exciter voltage source return node 42 decreases, conduction current in the trace 40 increases as a result of cumulative displacement current injection. Because the traces on the electronic circuitry 16 do not have zero impedance, the displacement currents cause voltage gradients to develop on the traces. This causes, for example, circuit ground to have significantly different potentials at different locations on the electronic circuitry 16. This is another way in which displacement current injection can cause noise voltages that impair reader performance.
As can be appreciated by those skilled in the art, there are numerous components and traces on the electronic circuitry 16 that may be injected by displacement currents as previously described. Because these many components and traces comprise a wide variety of impedances (large, small, linear, non-linear, real, complex, etc.) to the exciter voltage source return node 42, a wide range of noise voltages and currents exist, many of which occur in circuitry that is particularly sensitive. Many varied responses to noise exist as a result, which significantly impacts reader performance and stability. Because of these problems, readers may be unable to satisfy the needs of certain applications.
The adverse effects from displacement current injection may be reduced by significantly increasing the separation between the exciter electrode 14 and the electronic circuitry 16. This solution, however, is not acceptable for applications requiring compact readers, since very large separations are required to minimize the problem. Extensive layout and circuit modifications may be performed, but an iterative approach (trial and error) is usually required, and ultimately provides only marginal performance improvement. Extensive filtering schemes do not help significantly because displacement current injection occurs everywhere, not just in signal paths. The many and varied impedance paths in a reader can react to the displacement currents, introducing complex electromagnetic emission and susceptibility problems on top of performance problems. The approaches mentioned above usually add cost and complexity, and consume space, with little return in performance.
In addition, FIG. 2 illustrates several elements of an RFID device including exciter source impedance (Re) 31, exciter current (Ie) 33, and noise current (Ic) 37. Note that the sensitive circuitry 36, 38, 40 are placed on the side of the printed circuit board that is facing the exciter electrode 14. The sensitive circuitry 36, 38, 40 can also be directly exposed to the exciter electrode 14 if the printed circuit board is mounted at a variety of other angles relative to the exciter electrode 14, including a ninety-degree angle or perpendicular.
The exciter voltage source 30 generates a high voltage (Ve) and is connected through a source impedance (Re) 31 to an exciter electrode 14. The source impedance (Re) 31 is usually internal to the exciter voltage source 30 but the source impedance (Re) 31 may also be a device external, or in addition to, the exciter voltage source 30. The source impedance (Re) 31 may be resistive, capacitive, inductive, or any combination thereof. A current (Ie) 33 primarily flows from the exciter voltage source 30, through the source impedance (Re) 31 to the exciter electrode 14. The current (Ie) 33 is divided between a current that flows out of the exciter electrode 14 to ground (Ig) 35, a noise current (Ic) 37 that flows into the sensitive circuitry 36, 38, 40, and other less-significant paths not shown in FIG. 2. The noise current (Ic) 37 creates an undesirable noise voltage across the sensitive circuitry impedance, Rc.
One of the methods that may be used to attenuate the noise current (Ic) 37 present in the sensitive circuitry 36, 38, 40 is to add shielding around the sensitive circuitry 36, 38, 40, also known as a Faraday Box. The Faraday Box, however, is bulky and expensive, and is unacceptable for many hand-held RFID devices.
Thus, there exists a need to provide an apparatus and method for minimizing undesirable exciter displacement current (i.e., noise) in the electronic circuitry of RFID devices, and a low-cost solution which enables compact electric field RFID devices to function well and predictably in a wide variety of applications.